An (as of yet) unwritten rule that any food dropped on the floor may be picked up and eaten as long the person who dropped it states the number of seconds it has been on the floor and adds 'rule' on the end.

This is ever extendable for the length of time the food has been on the floor so can be used in any situation, especially if the dropped item is not easily accessible.

It is commonplace to wipe off some of the dust etc. before proceeding to eat

Person 1: Oh no I dropped my sweet under the table!
Person 2: No worries 25 second rule
Person 1: I thought that was just 3 seconds
Person 2: no it's the extendable second rule